An illumination system which uses a light source of said type is known from US 2007/0253733 A1. The cited document describes the use of the illumination system for a fluorescence microscope. An LED light source is positioned at a focal point of an elliptical mirror and emits its radiation into a hemisphere facing toward the mirror. The mirror reflects the incident radiation and focuses it onto a downstream optical system. Due to the retroreflection from the reflector, i.e. reflection of beams having an angle of incidence of less than 45°, it is impossible to prevent the LED light source and the mechanical structures required for retaining it from themselves obstructing the reflected light.
Because the light yield of a LED decreases with increasing temperature it is necessary to ensure that the heat generated during its operation will be dissipated in order to minimize the heating-up of the LED during operation. If a LED module is arranged at the focal point of a backward-reflecting reflector, the module cannot be cooled by an otherwise conventional heatsink because the latter would shadow an even greater part of the light reflected from the reflector. Instead of this, the heat must be conducted to the outside by means of the retainers. Even if the latter can be implemented in a very compact design in order to keep losses of the reflected light to an absolute minimum, they take up more room than a LED on its own and consequently lead to a shadowing of the reflected light. Even if the retainers are embodied as transparent, the reflected radiation incident thereon is affected and the optical efficiency of the arrangement reduced.